1. Field of the Invention
The invention relates to X-ray units that are used to generate a beam of X-rays towards a patient's body in order to take a picture of this part by various methods such as projection on a sensitive film in standard type radiology instruments or the reconstruction of a 2D or 3D image from measurements of X-radiation made at different angles in scanner type instruments.
In present technology, an X-ray unit is constituted (FIG. i) by an X-ray tube 10 and a casing 12 filled with an insulating and cooling fluid 14 in which said X-ray tube is placed. The casing 12 is opaque to X-rays except at one place 16, the place through which the radiation (arrow 26) generated by the X-ray tube is emitted. The casing 12 is provided with the electrical terminals 18 and 34 which feed from an electrical power supply externally connected to the X-ray tube. In addition holes (not shown) provided for circulation, if necessary, of insulating and cooling fluid 14 and the filling of such fluid with casing 12.
As also shown in FIG. 1, an X-ray tube comprises a filament type cathode 20 that emits an electron beam 22 towards an anode 24 or anticathode. The anode 24 is constituted by a material such as tungsten or molybdenum which emits the beam 26 of X-rays when it is bombarded by the electron beam 22 coming from the cathode 20. To obtain a high-energy electron beam, the electrons are accelerated by an intense electrical field created between the cathode 20 and the anode 24. To this end, the anode 24 is taken to a positive potential of several tens of kilovolts with respect to the cathode. This potential may exceed hundred kilovolts and reach two hundred kilovolts.
These high electrical potentials are supplied by special cables called high-voltage cables coming from a high-voltage power supply device which is placed at some distance from the casing. This distance may reach 30 meters in certain radiology instruments.
2. Description of the Prior Art
In FIG. 1, the X-ray tube is of the rotating anode type, which implies a motor constituted by a rotor 28 fixedly joined to the anode 24 and a stator 30 fixedly joined to the envelope 32 of the X-ray tube 10, these two elements 28 and 30 having to be electrically supplied by the terminal 34.
Furthermore, since the cathode 20 is of the filament type, it should also be electrically supplied by the terminal 18.
In the example of the standard type of X-ray unit according to FIG. 1, the casing is constituted by four parts 36, 38, 40 and 42 which are assembled together in an imperviously sealed way to form an enclosed chamber in which the insulating and cooling fluid circulates through means not shown in FIG. 1. It must be noted that, in certain embodiments, the two central parts form only one part.
In this casing 12, the X-ray tube 10 is kept in position by brackets or clamps such as those referenced 44 and 46 which are fixedly joined to said casing.
FIG. 2 is a simplified electrical diagram showing how the high-voltage power supply of the X-ray tube is generally implemented.
The anode 24 and the cathode 20 are connected respectively to a high-voltage supply generator 48, placed outside the casing 12, by so-called high-voltage cables 50 and 52. Such a generator 48 comprises a high-voltage supply device 54 which supplies DC high voltages and an inverter type high-frequency converter 56, which provides the device 54 with low-voltage pulse signals having a frequency of some tens of kilohertz starting from the supply voltage E of the AC mains system.
In view of the power values needed for the operation of the X-ray tube 101 this tube is, as described further above, placed in a casing 12 containing an insulating and cooling fluid. For the same reason, the high-voltage supply device 54 is placed in another casing 58 filled with an insulating and cooling fluid.
As can be seen very schematically in FIG. 2, the high-voltage power supply device 54, which is also called a "high-voltage pack", comprises a transformer 60, having only one primary winding 62 and several secondary windings 64.sub.1 to 64.sub.n. The primary winding 60 has two input terminals E1 and E2 that are connected to two output terminals of the high-frequency converter 56.
Each of the secondary windings 64.sub.1 to 64.sub.n is connected to a rectifier and filtering circuit represented schematically by a diode D and a capacitor C, and the secondary windings are connected to each other in such a way that their output voltages get added up to obtain the desired high voltage.
A single-pole supply voltage is obtained by placing the supply conductor of the cathode (cable 52) at the potential of the ground while a symmetrical two-pole supply voltage is obtained by placing the midpoint M of the secondary circuits at the potential of the ground.
A power supply device such as this for an X-ray tube comprising high-voltage cables 50 and 52 has the following drawbacks:
two casings containing insulating and cooling liquids are used, one 12 for the X-ray tube 10 and the other 58 for the high-voltage power supply device, and the result thereof is an increased cost of the X-ray unit; PA1 the high-voltage cables 50 and 52 are specially built, and are therefore costly; PA1 the parasitic capacitance of the high-voltage cables 50 and 52 is all the greater as the cables are long, thus limiting the speed of build-up of the high voltage on the X-ray tube; PA1 as a result of the complex movements of the support of the X-ray unit, the high-voltage cables 50 and 52 are subjected to major mechanical strains that lead to problems related to reliability, safety and economy.
It is an object of the present invention, therefore, to make a high-voltage power supply device for an X-ray tube that does not have the drawbacks due to the use of special high-voltage cables.
This object is achieved through the use of a high-voltage power supply device, the dimensions of which are such that it can be placed inside the casing of the X-ray unit and can thus be in the immediate vicinity of the X-ray tube to be supplied. Thus, there is no longer any need for special high-voltage cables.
Such a high-voltage power supply device has been described in the U.S. Pat. No. 5,003,452.
To enable this power supply device to be placed in the casing of the X-ray unit, it is necessary to modify said casing and, notably, to extend it, either on the cathode side or on the anode side or on both sides. Furthermore, certain devices and protection elements have to be provided for.